An Investigation into Transport Protocols and Data Transport ...

9.1. Transfer Tests Across Dedicated Private Wide Area Networks 210
1e+06
100000
TCP Slow Path
TCP Fast Path
10000
Frequency
1000
100
10
1
1 10 100 1000 10000
Number of Segments Acknowledged
Figure 9.16: Histogram of the Number of Segments acknowledged per ack under
a Dummynet Link of 600Mbit/sec and 40ms RTT.
obey delayed acking where an acknowledgment is generated for every other
data packet, then a dubious circumstance occurs. Similarly, if the reverse
path of the TCP data connection is congested, then acknowledgments may
be lost which will also generate a dubious event. Similarly, if ack packets are
reordered, a dubious event will also occur. As the testbed networks did not
have any reverse traffic, the latter two possibilities are unlikely.
Figure 9.16 shows the number of segments acknowledged by each ack. It
clearly shows that there is a high number of acks that acknowledge more than
just the two packets expected for delayed acking. Under such circumstances,
dubious conditions will be prevalent and calls to moderate_cwnd() will occur
and therefore goodput performance diminishes.
SACK Processing
The reductions of cwnd due to moderate_cwnd() do not account for the
double or triple reductions in ssthresh upon congestion which are evident
under the DataTAG tests. Should TCP Reno or NewReno be used for these
tests, the multiple reductions can be explained by the multiple drops per
congestion window which Reno and NewReno would require to exit from loss
recovery (See Section 4.5.3). However, as all tests were run with SACKs,